In recent years, broadband wireless systems have been put into practical use. These modulation/demodulation systems adopt a multi-carrier (OFDM or the like) method using many carrier waves in order to implement systems having a high frequency use rate.
In this case, an amplitude of a transmission signal instantaneously becomes very large. Since a power amplifier normally causes a linear distortion, its output signal is distorted. To make the distortion fall within a tolerable range, a power amplifier having a high peak output power needs to be used. Namely, a large back-off needs to be secured in order to maintain linearity, leading to a decrease in power efficiency. This is very inconvenient, especially for mobile terminals (the duration of a call becomes very short). Accordingly, it is desired to realize a power amplifier having an efficiency that does not decrease within a power level ranging from peak power to a relatively low power (within a wide dynamic range).
An effective method for operating a power amplifier using a semiconductor element at a high power supply efficiency is to reduce a loss caused within the element. Accordingly, the power amplifier normally needs to be operated so that a time integration of a product of a current and a voltage at an output terminal where an internal loss is caused within the element becomes small. To implement this, a saturation operation is effective. Some methods for efficiently extracting a wave having a modulated amplitude from an amplification element that is continuously performing a saturation operation have been developed and are widely known. The purpose of the saturation operation is to amplify a signal over an entire dynamic range of an amplifier. Since the entire dynamic range is used, a signal is amplified by using an entire operable range of an amplifier. This leads to an increase in power efficiency.
Representative circuits include LINC (Linear Amplification Using Nonlinear Components), a Doherty amplifier, envelope tracking (ET), and envelope elimination and restoration (EER). These circuits are summarized below.
FIGS. 1A to 1C respectively illustrate configurations of the representative amplifiers of the conventional techniques.
1. LINC (FIG. 1A): Two amplifiers are made to perform a saturation operation by being driven at a constant and large amplitude, and output powers are synthesized and output. The amplitude of each of the outputs is varied by changing a phase of each of the inputs.2. Doherty amplifier (FIG. 1B): Two amplifiers, such as a main amplifier and an auxiliary amplifier, are included. A signal that is delayed by λ/4 from an input of the main amplifier is input to the auxiliary amplifier, and an output of the main amplifier is delayed by λ/4 and synthesized with an output of the auxiliary amplifier. Normally, the main amplifier is operated in class A to AB, whereas the auxiliary amplifier is operated in class B. If input power is increased, the main amplifier starts a saturation operation earlier, and achieves a high efficiency.3. ET, EER (FIG. 1C): A power supply voltage is varied while causing the amplifier to perform a saturation operation, and a signal having a modulated amplitude is obtained as an output.
However, the above described conventional techniques have the following problems.
1. LINC: It is difficult to efficiently synthesize the outputs of the two amplifiers.
2. Doherty amplifier: A linear circuit having a small loss is used as a power matching circuit of the input/output of each of the amplifiers and as a power synthesizer of the outputs. Therefore, input/output impedances of the amplifiers and a phase/amplitude transfer characteristic from the input to the output change depending on the size of a signal. In particular, this change becomes significant if a bias current is reduced to increase efficiency in the case of low power. The linear power matching circuit and the power synthesizer of the outputs cannot cope with the change in the input/output impedance and the phase/amplitude transfer characteristic from the input to the output, and efficiency decreases if signal power varies. Moreover, a small amplifier that quickly achieves a high efficiency by being saturated at a low output power level in a wide power range where the amplifier operates and a large amplifier capable of outputting a high power are combined and used. If the sizes of elements used for the large and the small amplifiers are significantly different, it is possible that the elements of the small amplifier will be destroyed if power input to the large and the small amplifiers is the same.